Add preview release versioning and update channel support (#49414)

## Summary
- Publish scheduled `main` builds as GitHub prereleases while keeping
manual `main` runs as preview validation builds.
- Add an opt-in Settings switch for prerelease update checks; stable
updates remain the default.
- Use one MSI-safe version across bundles, MSI packages, binaries,
symbols, and package manifests.
- Prevent preview releases from triggering Microsoft Store, WinGet, or
public-symbol publication.
- Label preview builds explicitly in Settings, update notifications, and
What's New.

## Build intent

| Source | Trigger | Intent |
| --- | --- | --- |
| `main` | Scheduled | Publish a preview release |
| `main` | Manual | Validate a preview build without publishing |
| `stable` | Manual | Produce a stable release |
| Other branches | Any supported trigger | Produce a private validation
build |

## MSI-safe release versioning

Windows Installer compares only `major.minor.build` and ignores the
fourth version component. Preview and stable release builds therefore
use:

```text
major.minor.YDDDB.0
```

- `Y`: zero-based number of calendar years since `ReleaseTrainEpoch`.
- `DDD`: three-position calendar day of year.
- `B`: daily release sequence `1-9`.
- The fourth component is always `0`.

With `ReleaseTrainVersion=0.100` and `ReleaseTrainEpoch=2026-01-01`:

```text
0.100.2111.0   = July 30, 2026, release build 1
0.100.3659.0   = December 31, 2026, release build 9
0.100.10011.0  = January 1, 2027, release build 1
```

The allocator formats `DDD` as exactly three digits before converting
the MSI component to its numeric representation. Leading zeros may not
be displayed because Windows version components are numeric; decoding
remains positional:

```text
B   = component % 10
DDD = (component / 10) % 1000
Y   = component / 10000
```

`ReleaseTrainVersion` and `ReleaseTrainEpoch` are checked in under
`src/Version.props`. The epoch remains January 1 of the active epoch
year and advances on the first release-train minor change in a new year.

## Daily release counter

Azure DevOps persists the daily sequence server-side using a counter
keyed as `release-YYYYMMDD`.

- `main` and `stable` share the same daily counter.
- Other branches do not evaluate or consume the release counter.
- Failed or canceled `main`/`stable` runs may leave gaps.
- The build fails when the daily sequence exceeds `9`.
- The counter date and encoded `YDDD` date both use
`pipeline.startTime`.

Private branches retain independent `0.0.<extended-day><NN>.0`
validation versions.

## Update behavior
- Stable users continue to query GitHub's stable latest-release path.
- Users who explicitly enable preview updates can select newer GitHub
prereleases.
- Preview releases and notifications are labeled as PowerToys Preview.
- What's New separates preview entries from stable release history and
hides previews by default.

## Validation
- 17 Pester tests cover `main`, `stable`, private branches, year
rollover, epoch reset, monotonicity, override validation, sequence
limits, and date alignment.
- Version propagation verified `0.100.2111.0` in `Version.props` and all
affected AppX/MSIX manifests.
- Azure DevOps pipeline dry-runs succeeded for both `refs/heads/main`
and `refs/heads/stable`.
- The affected native version project builds successfully.
- PR CI is green for x64, ARM64, Command Palette SDK, dependency review,
telemetry detection, and CLA.

## Remaining end-to-end checks
- Install two locally or officially produced installers with consecutive
MSI-visible `YDDDB` versions and verify the upgrade preserves binaries,
package registrations, hardlinks, and shell integrations.
- On the first natural post-merge `main` or `stable` run, verify the
production counter value and resolved version in the release logs.
## Local GPO verification

Validated locally with the signed `v0.100.2171` build from Azure DevOps
build
[153961073](https://microsoft.visualstudio.com/Dart/_build/results?buildId=153961073).
These checks cover the administrative-template integration and Settings
behavior.

### Policy enabled: preview updates are disabled

With `PreviewUpdatesDisabled=1`, **Include prerelease updates** is
forced off and locked, and Settings displays the
managed-by-your-organization notice.

![PowerToys Settings with preview updates disabled by
policy](https://raw.githubusercontent.com/LegendaryBlair/PowerToys/df808630b04e65ba437081aff9401c4efd58e67f/.github/pr-assets/49414/gpo-policy-enforced.png)

### Policy removed: the user preference is preserved

After removing `PreviewUpdatesDisabled` and restarting PowerToys, the
previously selected preview-update preference is restored and editable.
The policy suppresses the preference without overwriting it.

![PowerToys Settings with the preview-update preference
restored](https://raw.githubusercontent.com/LegendaryBlair/PowerToys/df808630b04e65ba437081aff9401c4efd58e67f/.github/pr-assets/49414/gpo-preference-restored.png)

### Group Policy Editor

After importing the updated ADMX/ADML templates, **Disable preview build
updates** appears under **Microsoft PowerToys > Installer and Updates**.
The policy dialog documents that **Enabled** blocks preview updates,
while **Disabled** or **Not Configured** leaves the choice available to
the user.

![Disable preview build updates in Local Group Policy
Editor](https://raw.githubusercontent.com/LegendaryBlair/PowerToys/e9e5c12f4480ef895d263460a59982246b7654dc/.github/pr-assets/49414/group-policy-editor-policy-dialog.png)

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: ad8b7909-0472-4464-bdee-deaeca726f94
Copilot-Session: 8e04a72e-3b0f-4ac4-8156-d04ea9b8bb85

Directory.Build.props

@@ -18,7 +18,7 @@
     <AnalysisMode>Recommended</AnalysisMode>
     <_SkipUpgradeNetAnalyzersNuGetWarning>true</_SkipUpgradeNetAnalyzersNuGetWarning>
     <NuGetAuditMode>direct</NuGetAuditMode>
-    <IncludeSourceRevisionInInformationalVersion>false</IncludeSourceRevisionInInformationalVersion>    <!-- Don't add source revision hash to the product version of binaries. -->
+    <IncludeSourceRevisionInInformationalVersion>false</IncludeSourceRevisionInInformationalVersion>    <!-- Source commit is exposed explicitly via Version.props/version_gen.h. -->
     <PlatformTarget>$(Platform)</PlatformTarget>
     <RestoreEnablePackagePruning Condition=" '$(VisualStudioVersion)' == '17.0'">false </RestoreEnablePackagePruning>
 
@@ -74,7 +74,9 @@
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(MSBuildProjectExtension)' == '.csproj'">
-    <Version>$(Version).0</Version>
+    <VersionBuildSuffix Condition="$([System.Text.RegularExpressions.Regex]::IsMatch('$(Version)', '^\d+\.\d+\.\d+$'))">.0</VersionBuildSuffix>
+    <VersionBuildSuffix Condition="'$(VersionBuildSuffix)' == ''"></VersionBuildSuffix>
+    <Version>$(Version)$(VersionBuildSuffix)</Version>
     <RepositoryUrl>https://github.com/microsoft/PowerToys</RepositoryUrl>
     <RepositoryType>GitHub</RepositoryType>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>